This invention relates to electronic alarm devices such as are used in the fire protection industry. More particularly, it relates to piezo electronic alarm horns which use piezo crystal sounders, and particularly in alarm systems of the type in which an alarm condition is indicated by a reversal of the polarity on the d.c. power supply to the horn.
There has developed a need to improve alarm horns and particularly a need to improve their electrical efficiency while at the same time reducing their cost. These improvements, of course, must not jeopardize the applicable standards for constancy of sound level output under varying supply voltages. Since a higher electrical efficiency will result in a lower operating line current, the number of units a particular fire alarm control panel can power will increase. If at the same time one can design a unit which will operate on both the standard 12 volt d.c. system as well as the standard 24 volt d.c. system, then the units can also be made less expensive.
Underwriters Laboratory specifications require that operation of these devices must continue when the supply voltage drops by as much as 80% of the nominal value and also when it rises to 110% of the nominal value. Thus, in the lower voltage range the unit must operate between 8 and 13.2 volts, and in the upper voltage range it must operate in the range between 16 and 26.4 volts. In order to cover both ranges the device must, therefore, be operable over the range between 8 and 26.4 volts. It is also a requirement of UL specifications that the sound output of the horn cannot decrease by more than 3 db over the range of 8-26.4 volts d.c.
It was determined that costs could be reduced if it was possible to use piezo sounders to provide audible output in electronic horns of this type, for they are much cheaper than electromagnetic horns. Typically, piezo sounder have been directly connected to oscillators as the source of energy, as is shown in U.S. Pat. No. 4,558,305 issued to Black et al.; or they have been connected as a self-excited audio transducer, as shown in U.S. Pat. No. 4,594,573, issued to Yoshino et al. Such arrangements, however, tend to produce a single frequency sound which is unsatisfactory for fire alarm systems.
It is an object of this invention to provide a circuit design for an inexpensive electronic horn which will be useful in both 12 volt and 24 volt systems, and one which will be very efficient electrically so as to minimize the current requirements for the power supply. It is a further object of this invention to maintain a constant and sufficient sound output having a suitable range of overtones even though the supply voltage may vary between 8 and 26.4 volts d.c.